Can't get Blender to render smoke
Select the Flow object and make sure that the Flow Type: is set to Fire + Smoke. It is likely just set to Fire. If this doesn't help, consider uploading a .blend so someone can locate the problem more easily.

Edit:

Got it! The issue was in your nodes. I'm not quite sure what the issue was, but I replaced the current smoke nodes with this, which I plugged into the mix node that the other thing was previously plugged into:



The value in the multiply node affects how dense the smoke is. Here is the fixed .blend file: